1. Field of the Invention
The present invention relates to a piezoelectric element, a liquid droplet discharging head, a liquid droplet discharging device, an image forming apparatus, and a manufacturing method of a piezoelectric element, and more specifically, the present invention relates to a piezoelectric element, a liquid droplet discharging head, a liquid droplet discharging device, an image forming apparatus, and a manufacturing method of a piezoelectric element used in an image forming apparatus such as a printer, a facsimile machine, a copier, a plotter, or a multifunction peripheral in which a plurality of these functions are combined.
2. Description of the Related Art
There is a device for forming images, such as a printer, a facsimile machine, a copier, a plotter, or a multifunction peripheral in which a plurality of these functions are combined. This device is provided with a liquid droplet discharging head for discharging liquid droplets of, for example, ink. This device forms images by discharging ink droplets from the liquid droplet discharging head while conveying a recording medium, and causing the ink droplets to adhere to the recording medium. As a liquid droplet discharging head, there is known a piezoelectric type liquid droplet discharging head provided with a piezoelectric element (see, for example, Patent Documents 1 through 3). In the piezoelectric type liquid droplet discharging head, a plurality of liquid chambers are disposed, respectively corresponding to a plurality of arranged nozzles for discharging ink droplets. At least one of the walls of the liquid chamber is formed by a vibration plate. The vibration plate is caused to deform by a piezoelectric element so that the volume of the liquid chamber changes and ink droplets are discharged.
In order to improve the reliability of the liquid droplet discharging head, the piezoelectric element needs to be prevented from breaking, and the operating life needs to be extended. The breaking of the piezoelectric element is largely classified into two types, namely, cracking and insulation breakdown. Insulation breakdown is relatively easy to avoid by optimizing the design in forming electrodes/piezoelectric elements and in forming a protection film, apart from irregular cases where foreign objects enter from outside.
Patent Document 1 discloses the following technology for the purpose of providing a piezoelectric element having high reliability. That is to say, at least part of a side surface 22 in the short-side direction of a piezoelectric body layer 20, is an irregular surface 23, and the width Wy in the short-side direction of the piezoelectric body layer 20 changes according to the irregular surface 23, from a second electrode 30 toward a first electrode 10.
Patent Document 2 discloses the following technology for the purpose of providing a piezoelectric actuator of an inkjet print head and a method of forming the same. That is to say, a lower electrode formed on top of a vibration plate constituting a top wall of a pressure chamber; a piezoelectric film formed at a position corresponding to the pressure chamber on the lower electrode, such that a space is formed between the surrounding part of the piezoelectric film and the lower electrode; and an upper electrode formed on the piezoelectric film for applying a voltage to the piezoelectric film, are provided. The piezoelectric film may be provided such that the side surface of the piezoelectric film substantially forms a right angle with respect to the top surface of the lower electrode, instead of forming a space at the surrounding part.
Patent Document 3 discloses the following technology for the purpose of providing a liquid jetting head, a liquid jetting device, and an actuator device by which durability is improved. That is to say, a first electrode 60 is formed along a plurality of active parts 320, and at the edge part 66 of a first area 65 corresponding to the active parts 320 of the first electrode 60, there is provided a thick film part 68, which is thicker than a thin film part 69 and thicker than a second area 67 between the first electrode 60 and the active parts 320.
The above conventional technologies of Patent Documents 1 through 3 have similar intentions to that of the present invention in that the piezoelectric element is shaped to have a characteristic feature to increase the reliability and to reduce the concentration of stress. However, the above conventional technologies are different from the present invention of attaining a piezoelectric element in which stress concentration hardly occurs even by being vibrated and which is highly resistant to cracks.    Patent Document 1: Japanese Laid-Open Patent Publication No. 2012-192541    Patent Document 2: Japanese Laid-Open Patent Publication No. 2010-132008    Patent Document 3: Japanese Laid-Open Patent Publication No. 2010-228276
Based on the contents described with reference to Patent Documents 1 through 3, the following is considered. Unlike insulation breakdown, cracks may occur due to various factors and at various timings. Therefore, screening is difficult, and drastic measures for preventing cracks are needed.
Cracks are often caused by concentration of stress due to structural factors, and therefore the structure needs to be formed such that stress is hardly concentrated.
In the case of a discharging method of deforming a piezoelectric film or a piezoelectric body layer that is a thin film having a thickness of several μm (so-called vent mode), the edge part of PZT is apt to receive stress due to the deforming, and is apt to receive stress due to the concentration of an electric field and differences in the intensity of the electric field, at the electrode edge. Therefore, for the purpose of improving reliability, the piezoelectric element needs to be designed from a new, breakthrough viewpoint of alleviating the concentration of stress at the edge part of PZT. Note that PZT forms the piezoelectric layer of the piezoelectric element, and means “a solid solution of lead zirconate (PbZrO3) and titanic acid (PbTiO3)” or “lead zirconate titanate”.